1. Field of the Invention
The present invention relates to hearing aids. The invention more specifically relates to an output stage for a hearing aid and to a method of driving an output transducer for a hearing aid. The invention, still more specifically, relates to an output stage for driving the output transducer in a hearing aid.
2. The Prior Art
U.S. Pat. No. 5,578,963 discloses an improved class B driver-circuit (a so-called push-pull driver). This is an analog driver that may be used in hearing aids with a digital processor, provided the processor is followed by a DAC (Digital-Analog Converter) for transforming digital signals emanating from the signal processor into analog output signals that control the output driver.
U.S. Pat. No. 5,878,146 relates to a digital hearing aid with a special type of output stage known as a Sigma-Delta converter. In a Sigma-Delta converter a Sigma-Delta modulator is controlled by a digital input and produces a high-speed pulse-modulated binary signal, which is fed to a series-connected low-pass filter, wherein the signal is converted into a low-frequent analog signal. In one embodiment of this converter, the low-pass filtering of the high-bitrate output signal from the Sigma-Delta (S-D) converter is provided by the driver coil in the output transducer, the transducer driver coil performing in this way a dual-function. When the output transducer is relied on for low-pass filtering, the converter may be said to comprise both the modulator, the output driving circuitry and the output transducer. Another way of describing this design would be to say that the output stage is a direct digital output stage, since no DA-conversion, in the ordinary sense, is performed in the hearing aid. In such a device, the DAC is replaced by an S-D modulator, which transforms the digital output from the signal processor into a binary signal suitable for driving the digital (switching) output driver.
U.S. 20020186155 discloses a digital amplifier with a stage for converting an analog input, and with an H-bridge controller, wherein the two halves of the bridges are controlled independently of one another in order to create a three-state condition instead of the conventional two states.
It is a disadvantage with this system that power consumption it relatively high for low amplitude input signals. This is due to the fact that when the high-bit-rate output from a Sigma-Delta modulator is utilized in a push-pull driver, e.g. a FET-based driver, a low-amplitude signal is represented by a binary signal that continually switches between two states at a high rate, causing frequent re-charging of the transistors in the output driver. The high power consumption is a disadvantage due to the reduced battery life-time.
WO03/047309-A1 discloses an S-D DAC. In this prior art system, which is suggested utilized in hearing aids, an S-D modulator, capable of operating with three output states, is utilized. This three-state signal controls a push-pull output driver—a so-called H-bridge, whose output is filtered in a lowpass filter.
Due to a rather complex design, this system is not suitable for use in a hearing aid.
The prior art also has examples on output drivers that utilize more than two states in the driver circuitry. The advantage of this is a reduction in power consumption in the driver stage. It has never been suggested, though, that corresponding three-level control signals can be derived from a simple 1-bit S-D modulator.